Lipid composition of gallbladder bile, bile acid pool size and sex hormone levels were measured in 66 Pima Indians aged 9 to 21 years in order to determine the age of onset of lithogenic bile in this population noted for a high prevalence of cholesterol gallstones. Bile acid pool size is measured by isotope dilution using deuterated chenodeoxycholic acid, and gallbladder bile is obtained by duodenal siphonage after stimulation of gallbladder contraction. Highly saturated bile was not prevalent among Pimas under the age of 13, but bile saturation increased significantly in both sexes during pubertal growth and development. Bile saturation was 15 per cent higher in females than males. Bile acid pools increased with age in the young men, but not in women. Bile cholesterol saturation correlated with obesity and urinary estrogen excretion. Highly saturated bile may be present for several years before the onset of cholesterol cholelithiasis.